Apparatus for the manufacture of tubes from thermoplastic materials

摘要

In the manufacture of tubes by extruding a thermoplastic material through an annular orifice formed by an outer casing and a core, any elements which project into the annular passage, such as supports for the core, are locally heated. This prevents streaks of reduced thickness or stability in the finished tube which might otherwise be formed by local cooling. The heating may be by electrical resistance or by circulation of heated fluids. It is preferred to introduce the thermoplastic material into the extrusion head at a temperature considerably higher than its softening point and to allow a decrease of temperature, e.g. by means of cooling fins, so that it leaves the nozzle at a temperature nearer its softening point. As shown in Fig. 1, the extrusion nozzle consists of a core a carried by supports b from an outer casing c. A heating system d is arranged near the supports b. Alternatively the supports b may be made hollow and electric heating elements embedded therein. In another example, Fig. 4, the outer casing is made in two parts c1 and c2 which are flanged and bolted together. The supports b for the core a are carried by the upper half c1 and adjacent to the supports b an annular groove h is formed and electric heating elements i are embedded therein, the elements being retained in position by an insulating ring k clamped between the two flanges. Suitable thermoplastic materials are polyethylene, polystyrene, polyvinyl chloride, polymeric chlorotrifluorethylene, superpolyamides or plasticized cellulose acetate.